Mondas is a planet from where the Cyberman originate. Mondas is Earths twin planet but it left the orbit to return later in the episode The Tenth Planet. Mondas blew up in the end, the inhabitents survive on spaceships as they travel from planet looking to conquer and survive. Given the harsh atmosphere that would`ve existed on Mondas, the inhabitents enchanced their bodies, eventually turning into Cybernetic beings. In the alternate universe, Cybermen do not originate from Mondas, they originate on Earth as one mans attempt to extend human life.